Police closed Westminster Bridge in London and attempted to contain thousands of Tamil protesters who surrounded parliament last night demanding an immediate ceasefire in Sri Lanka.

One man was rescued after jumping into the Thames. Four RNLI lifeboats were on standby in the river after reports that demonstrators had threatened to throw themselves off the bridge

Scotland Yard estimated that up to 3,000 people occupied streets around the Houses of Parliament. Westminster tube station was closed by police.

The demonstration was against the killings in northern Sri Lanka where government forces have encircled the remnants of the separatist Tamil Tigers and thousands of civilians. The demonstrators went ahead without permission.

Under the Serious Organised Crime and Police Act 2005, it is an offence to stage an unauthorised demonstration within the vicinity of parliament. The government has said it is reviewing the law.

The protesters accused the Sri Lankan army of using cluster bombs and killing 5,000 civilians in the past four months.

Large numbers kept up the protest into the early hours. Witnesses said families, including children, set up tents and refused to be moved. "The police are not letting anyone through," one witness, Emma Gritt, said. "People are setting up tents. Police have blocked the bridge off."

A Scotland Yard spokeswoman said there had been no arrests. She confirmed Westminster Bridge had been closed. "There's loose containment," she said. "People can leave and join. Officers are negotiating to find out when it's going to end."